Degeneration, a biological term used to describe those not unfrequent cases where an entire organism falls below the structural level of its young stages, or where an organ in the same way loses its fullness of function, and becomes more or less atrophied, abortive, and simplified. Thus many parasitic worms, crustaceans, &c. are emphatically simpler than their free-swimming larvæ, and the sessile adult Ascidian shows only traces of the vertebrate characters which are plain enough in the active young. Thus, too, a crustacean which starts with a well-developed eye, may exhibit the gradual loss of this on assuming a dark habitat. The term is best confined to cases where a level of structure exhibited during early life is more or less lost in the adult. Degeneration must be distinguished (a) from occasional abortion, (b) from Reversion (q.v.) to an ancestral type, and (c) from the occurrence of rudimentary and undeveloped organs where a character possessed by ancestral types remains more or less undeveloped, or shows itself only in embryonic life. Degeneration may be due to the environment, or to cessation of function, or to some more subtle constitutional cause. Absence of food, heat, light, &c. may mean the absence of the necessary stimulus for the growth and maintenance of the organs, or superfluity of food may cause one system to preponderate over others. Nor can it be doubted that cessation of function checks the food-supply to a given organ, and in other ways helps to bring about its degeneration. But on the other hand, some less obvious cause—the fatigue of early life, a constitutional sluggishness, &c. may share in conditioning degeneration, as in the case of the majority of the Tunicata. Weismann and others, however, would explain degeneration by what they call the non-operation of natural selection. On this view, organs are not only developed but maintained by natural selection, and if it happen that an organ is no longer an advantage in this struggle for existence (e.g. eyes in dark caves), then natural selection no longer maintains that organ, and it disappears in the course of generations. Weismann applies this ultra-Darwinian conception especially to cases which might be called non-development rather than actual degeneration—e.g. to the slightly developed wings of the Apteryx. Most cases of degeneration properly so called appear hardly to require his subtle explanation, but find a sufficient one in the nature of the environment, in the effects of stopped function, and in the constitution of the organism. The theory of the degeneration of man from a high state (see ADAM, FALL) has been superseded by the belief in a development from low savagery (see ANTHROPOLOGY, ARCHEOLOGY).
See Ray Lankester. Degeneration (1880); and Weismann, Ueber den Rückschritt in der Natur (1887). Max Nordau, Degeneration (trans. 1895). See also, for instances, ASCIDIANS, CAVE-ANIMALS, CRUSTACEA, INSECTS, PARASITISM; also ENVIRONMENT, EVOLUTION, &c.